open_robot_actuator_hardware VS ODrive

So on the budget side of thing, take a look at the ODRI robot for a starting point. https://github.com/open-dynamic-robot-initiative/open_robot_actuator_hardware

https://open-dynamic-robot-initiative.github.io/ They have mechanical designs and code for torque controlled quadruped (so something like Spot, not like "toy" quadrupeds with position controlled servos). They also say it's cheap, but while 4000€ might be little for industry or academia, it is probably not insignificant amount for an individual. But there are many other versions, check them out.

ODrive perhaps?

Well I dunno, maybe see how ODrive does it

Then maybe just hook everything to an Odrive and maybe add an Arduino or something to tell it what to do - although this also means you'll probably want an encoder at least on the pedalling motor to get accurate position/speed readings at low speed, since pedalling cadence is typically in the 60-90RPM range

If you want speed, use servomotors

This video may interest you, maybe also check this one and check out simpleFOC and/or ODrive

ODrive time?

Look at using some RC brushless motors driven from an ODrive board

You can probably replicate your own with appropriately sized RC brushless motors driven from ODrive boards. The boards can be linked together via CAN bus to communicate with your Arduino equipped with a CAN bus shield.

Thanks for the link! I know about simplefoc, the problem is the hardware. Simplefoc focuses on low wattage motors, e.g. a gimbal motor for a camera. For high wattage application, there's not much hardware available, and none that is affordable. There's odrive, which will set you back ~300$ just for the controller. Other than that, you'll have to go to traditional robotics companies.